The present invention relates to the configuration of a semiconductor device having a transistor. The invention also relates to the configuration of an active matrix display device including a semiconductor device having a thin film transistor (hereafter, it is denoted by TFT) fabricated on an insulator such as glass and plastics. In addition, the invention relates to an electronic device using such the display device.
In recent years, the development of display devices using light emitting elements including electroluminescent (EL) elements has been conducted actively. The light emitting element has high visibility because it emits light for itself. It does not need a back light that is needed in liquid crystal display devices (LCD), and thus it is suitable for forming to have a low profile and has nearly no limits to the field of view.
Here, the EL element is an element having a light emitting layer that can obtain luminescence generated by applying an electric filed. The light emitting layer has light emission (fluorescence) in returning from the singlet excited state to the ground state, and light emission (phosphorescence) in returning from the triplet excited state to the ground state. In the invention, the light emitting device may have any light emission forms above.
The EL element is configured in which the light emitting layer is sandwiched between a pair of electrodes (an anode and a cathode), forming a laminated structure in general. Typically, the laminated structure of the anode/hole transport layer/emissive layer/electron transport layer/cathode is named, which was proposed by Tang et al., Eastman Kodak Company. This structure has significantly high luminous efficiency, which is adapted to many EL elements now under investigation.
Furthermore, there are the other structures laminated between an anode and a cathode in the order of the hole injection layer/hole transport layer/light emitting layer/electron transport layer, or hole injection layer/hole transport layer/light emitting layer/electron transport layer/electron injection layer. As the EL element structure used for the light emitting device in the invention, any structure described above may be adapted. Moreover, fluorescent dyes may be doped into the light emitting layer.
In the specification, the entire layers disposed between the anode and the cathode are collectively called the EL layer in the EL element. Accordingly, the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, and the electron injection layer are all included in the EL element. The light emitting element formed of the anode, the EL layer, and the cathode is called EL element.
FIGS. 2A and 2B depict the configuration of a pixel in a general light emitting device. In addition, as the typical light emitting device, an EL display device is exemplified. The pixel shown in FIGS. 2A and 2B has a source signal line 201, a gate signal line 202, a switching TFT 203, a driving TFT 204, a capacitance element 205, a current supply line 206, an EL element 207, and a power source line 208. The P-channel type is used for the driving TFT 204 in FIG. 2A, and the N-channel type is used for the driving TFT 204 in FIG. 2B. The switching TFT 203 is a TFT that functions as a switch in inputting video signals to the pixel, and thus the polarity is not defined.
The connection of each part will be described. Here, the TFT has three terminals, the gate, the source and the drain, but the source and the drain cannot differ from each other distinctly because of the structure of the TFT. Therefore, in describing the connection between the elements, one of the source and the drain is denoted by a first electrode, and the other is a second electrode. When the description is needed for potential of each terminal (the gate-source voltage of a certain TFT) about turning on and off the TFT, the source and the drain are denoted.
Furthermore, in the specification, the TFT being on is the state that the gate-source voltage of the TFT exceeds the threshold and current is carried between the source and the drain. The TFT being off is the state that the gate-source voltage of the TFT drops below the threshold and current is not carried between the source and the drain.
The gate electrode of the switching TFT 203 is connected to the gate signal line 202, the first electrode of the switching TFT 203 is connected to the source signal line 201, and the second electrode of the switching TFT 203 is connected to the gate electrode of the TFT driving TFT 204. The first electrode of the driving TFT 204 is connected to the current supply line 206, and the second electrode of the driving 204 is connected to the anode of the EL element 207. The cathode of the EL element 207 is connected to the power source line 208. The current supply line 206 and the power source line 208 have the potential difference each other. Moreover, to hold the gate-source voltage of the driving TFT 204, a certain fixed potential, the capacitance element 205 may be disposed between the gate electrode of the driving TFT 204 and the current supply line 206, for example.
When a pulse is inputted to the gate signal line 202 to turn on the switching TFT 203, video signals having been outputted to the source signal line 201 are inputted to the gate electrode of the driving TFT 204. The gate-source voltage of the driving TFT 204 is determined in accordance with the potential of the inputted video signals, and the current carried between the source and drain of the driving TFT 204 (hereafter, it is denoted by drain current) is determined. This current is supplied to the EL element 207 to emit light.